1. Field of the Invention
The present invention relates to a method for controlling the point of time when a bypass mode operation is begun, based on the format of pulse code modulation (PCM) data, and more particularly to a method for controlling the point of time, when a bypass mode operation is begun, based on a determination carried out in a switching unit, coupled to the vocoder, for the format of PCM data sent from the vocoder, in order to reduce errors generated due to a difference between the points of time when respective bypass mode operations of outgoing-end and incoming-end vocoders are begun.
2. Description of the Prior Art
A number of research efforts have been made to provide a method for improving speech quality to obtain improved call services in mobile-to-mobile (M-to-M) communications carried in a switching unit of a communications system, in particular, a digital mobile communications system.
FIG. 1 is a flow chart illustrating a bypass mode operation executed in a vocoder in accordance with a conventional call processing method. As shown in FIG. 1, the bypass mode operation involves an M-to-M recognition step 10 for recognizing a bypass command contained in an alarm, received at an optional switching unit, informing of the request for a call service, and recognizing the request for M-to-M communications, an incoming step 20 for receiving, at an incoming-end mobile station, a call signal sent from the switching unit after the execution of the M-to-M recognition step 10 and beginning a bypass operation in a vocoder associated with the incoming-end mobile station, and an outgoing step 30 for beginning a bypass operation in a vocoder associated with the outgoing-end mobile station after the execution of the incoming step 20, thereby enabling M-to-M communications.
Now, a conventional method for controlling the point of time, when the bypass mode operation of each vocoder is begun, in a call processing procedure.
In accordance with the conventional method, when a user sends a signal for requesting M-to-M communications to an associated switching unit, the switching unit recognizes a bypass command and M-to-M communications request data contained in the alarm informing of the request for a call service (M-to-M recognition step 10). Accordingly, the switching unit sends desired messages to outgoing-end and incoming-end mobile stations, respectively.
When the incoming-end mobile station receives the message from the switching unit, the incoming-end vocoder begins its bypass operation (incoming step 20). Thereafter, the outgoing-end vocoder begins its bypass operation. Thus, M-to-M communications can be enabled.
In accordance with the above-mentioned method adapted to control the bypass mode operations of vocoders for a call processing, normal communications between outgoing-end and incoming-end mobile stations can be enabled only when the vocoder associated with the outgoing mobile station begins a bypass mode operation within 20 to 40 msec after the vocoder associated with the incoming mobile station begins a bypass mode operation. For this reason, where the difference between the points of time when respective bypass mode operations are begun is undesirably lengthened due to a failure or error generated in the system, the vocoder associated with the outgoing mobile station may operate erroneously.
U.S. Pat. No. 5,491,719 describes a technique for reducing the generation of errors during transmissions of PCM data.
This patent discusses a system for handling data errors on a cellular communications system PCM link in order to reduce data errors generated during transmissions of PCM data. The system includes a switching unit and a base station. The switching unit includes means for coding speech signals, and a formatter for producing formatted data to be transmitted to the base station. The formatted data includes data to be actually transmitted, and an error detection code. The switching unit also includes means for detecting an error from a signal received from the base station and handling the detected error. The base station includes means for detecting an error from data received from the switching unit and handling the detected error, a channel encoder, a data transmitter for modulating data to be transmitted and transmitting the modulated data, a data receiver for receiving data from the switching unit and demodulating the received data, a channel decoder, and a formatter for producing formatted data to be transmitted to the switching unit, the formatted data including data to be actually transmitted and an error detection code.
In accordance with the system disclosed in the '719 patent, data is transmitted while including an error detection code by use of an error detection code generator for generating the error detection code for encoded speech and data signals and a formatter for formatting the error detection code and data to be transmitted, thereby producing formatted data to be finally transmitted. Upon receiving the transmitted data, it is determined whether or not an error is generated, based on the error detection code contained in the transmitted data. Thus, detection and handling of errors generated are achieved.
The system of U.S. Pat. No. 5,491,719 has an effect capable of transmitting data at a higher accuracy than PCM data. However, this system only reduces errors involved in data itself to be transmitted. The problem with this system is that it cannot reduce an error associated with the difference between the points of time when respective bypass mode operations are begun.